Five Stars year-end balance in accounts receivable is $2,000,000. The allowance for uncollectible accounts had a beginning-of-year credit balance of $30,000. An aging of accounts receivable at the end of the year indicates a required allowance of $38,000. If bad debt expense for the year was $40,000, what was the amount of bad debts written off during the year? (Hint: t-account)

I know that the answer is $32,000 but I dont quite understand why. Could someone explain?